Interference in a mobile terminal can be created by internal components or units. The smaller the mobile terminal or a transceiver unit is, the larger is the risk for internally generated interference. One typical example of internally generated interference is the interference caused by a clock signal leaking into the Radio Frequency (RF) parts of the transceiver unit. The harmonics of the clock signal extend into the RF domain and might therefore interfere with the received signal. This is especially a problem at low power levels of the received signal.
Sensitivity is a key parameter for receivers due to the fact that the sensitivity level is related to the coverage for a mobile terminal. Hence, from an operator point of view, there is a selling argument for mobile terminals or modems having a good sensitivity.
The problem with spurious signals in the receiver band, originated from a baseband clock signal, to mention one example, is typically mitigated by proper layout design of different blocks in the baseband and in the radio chip as well as on higher level routing of these blocks. However, based on practical experience such signal leakage can hardly be completely removed by layout design only. There can hence remain some residual leakage causing interference.
One solution of this problem according to prior art is to introduce a narrow-band notch filter in the time domain for filtering out the spurious signal. Since the clock frequency and the frequency of potential harmonics are known, narrow-band or notch filters might be introduced into the receiver chain.
This solution, however, has a drawback in that it requires a sampling rate in the order of 10 MHz of the received signal in combination with the need for a plurality of filter taps for establishing a sufficiently narrow notch filter, since the leakage signal is typically only about 100-1000 Hz wide. Such a filter is typically very expensive in terms of size, cost, and power or current consumption.
Therefore, there is a need for methods and arrangements improving the receiver performance close to the reference sensitivity level in the case of having harmonics from the baseband clock leaking into the receiver chain that do not require costly notch filters that consume considerable power.